Technical Release 210-60 Earth Dams and Reservoirs...TR 210-60 Earth Dams and Reservoirs i...

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Conservation Engineering Division

March 2019

Technical Release 210-60 Earth Dams

and Reservoirs

TR 210-60 Earth Dams and Reservoirs

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ACKNOWLEDGEMENTS This document updates the previous version of Technical Release No. 60, “Earth Dams and Reservoirs,” dated 2005. Tom Brown, Geotechnical Civil Engineer (retired), National Design, Construction, and Soil Mechanics Center (NDCSMC), Fort Worth, TX, under the direction and guidance of Steve Durgin, National Design Engineer, Conservation Engineering Division (CED), Washington, DC, led a team that prepared and reviewed updates to this technical release.

Numerous people provided source information as well as expert reviews and comments. NRCS acknowledges the efforts of the following individuals in developing this updated document: John Andrews, State Conservation Engineer, NRCS Colorado; Yasmin Bennett, Civil Engineer, NRCS South Carolina; Ruth Book, State Conservation Engineer, NRCS Illinois; Larry Cantrell, Civil Engineer, NRCS South Carolina; John Chua, Design Engineer, NRCS Arizona; Dennis Clute, Construction Engineer (retired), NDCSMC, Fort Worth, TX; Amanda Deans, Civil Engineer, NRCS Minnesota; Andy Deichert, State Conservation Engineer, NRCS West Virginia; Ben Doerge, Geotechnical Civil Engineer (retired), NDCSMC, Fort Worth, TX; Larry Fragomeli, Structural Engineer, NDCSMC, Fort Worth, TX; Jon Fripp, Stream Mechanics Civil Engineer, NDCSMC, Fort Worth, TX; Joe Gasperi, Geologist, NRCS Washington; Jared Gelderman, Assistant State Conservation Engineer, NRCS South Dakota; Alan Goble, Civil Engineer, NRCS Kentucky; Diane Guthrie, State Conservation Engineer, NRCS Georgia; Claudia Hoeft, National Hydraulic Engineer, CED, Washington, DC; Gregg Hudson, Geologist, NDCSMC, Fort Worth, TX; Jo Johnson, National Geologist, CED, Washington, DC; Jody Kraenzel, Civil Engineer, NDCSMC, Lincoln, NE; Paul Larson, Civil Engineer, NRCS Kansas; Mathew Lyons, State Conservation Engineer, Virginia; Jeff McClure, Geologist, West Virginia; Lee Ann Moore, Design Engineer, NDCSMC, Fort Worth, TX; Don Murray, Agricultural Engineer, NRCS Pennsylvania; Christian Osborn, State Conservation Engineer, NRCS Iowa; Tim Ridley, Dam Safety Engineer, NRCS West Virginia; Jose Rosado, Civil Engineer, NRCS Arizona; Larry Schieferecke, Civil Engineer, NRCS Kansas; Ben Smith, Hydraulic Engineer, NRCS Connecticut; John Van Leeuwen, Structural Engineer, NDCSMC, Fort Worth, TX; Ana Vargo, Geologist, NRCS Colorado; Carlos Velasquez, Hydraulic Engineer, NRCS California; Karl Visser, Hydraulic Engineer, NDCSMC, Fort Worth, TX; Dave Wolff, Agricultural Engineer, NRCS Colorado; Ken Worster, Design Engineer, NDCSMC, Fort Worth, TX; and Rod Yeoman, Design Engineer, NRCS Ohio.

Deborah Young, Writer-Editor, NRCS, Madison, MS; and Wayde Minami, Writer-Editor, NRCS, Beltsville, MD, prepared the document for publication.


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PREFACE This technical release (TR) describes design procedures and provides minimum requirements for planning and designing earth dams and associated spillways. It provides uniform criteria for earth dams and reservoirs. NRCS plans, designs, and constructs complex dams under widely varying conditions and it is essential to construct these dams with uniform criteria to ensure consistent performance. NRCS periodically revises this document to incorporate new experience, materials, and knowledge.

This TR references numerous NRCS and industry documents. Designers should use the latest version of these references.

This TR applies to—

• All low hazard potential dams with a product of effective storage times the effective height of the dam of 3,000-acre-feet2 or more.

• Dams more than 35 feet in effective height.

• All significant and high hazard-class dams.

This TR provides requirements as maximum or minimum limits that may not satisfy design criteria for all sites. In some cases, problems may arise that this document does not address. Where the state of practice has advanced or where proven solutions are not available, designers may need to evaluate alternate procedures before developing and selecting the best solutions. To ensure satisfactory performance, experience, State laws and regulations, investigations, analysis, expected maintenance, environmental considerations, or safety laws may dictate criteria that are more conservative.

This edition of the TR incorporates all previously issued revisions and amendments, as well as significant changes in Parts 2, “Hydrology”; 4, “Geologic and Geotechnical Considerations”; 5, “Earth Embankments and Foundations”; 6, “Principal Spillways”; and 7, “Auxiliary Spillways.”


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Table of Contents PART 1 – GENERAL ......................................................................................................................... 1-1

Dam Classification ............................................................................................................................................................................ 1-1 Classes of Dams ................................................................................................................................................................................. 1-1 Peak Breach Discharge Criteria .................................................................................................................................................. 1-2 Cut Slope Stability ............................................................................................................................................................................ 1-3 Reservoir Conservation Storage ................................................................................................................................................. 1-3 Joint Use of Reservoir Capacity ................................................................................................................................................... 1-3 Visual Resource Design .................................................................................................................................................................. 1-4 Safety and Protection ...................................................................................................................................................................... 1-4 Water Supply Pipes .......................................................................................................................................................................... 1-4 Utility Cables and Pipelines .......................................................................................................................................................... 1-5 Streamflow Diversion during Construction ........................................................................................................................... 1-5

PART 2 – HYDROLOGY .................................................................................................................... 2-1 Introduction ....................................................................................................................................................................................... 2-1 Principal Spillway Design Hydrographs .................................................................................................................................. 2-1 Auxiliary Spillway and Freeboard Hydrographs .................................................................................................................. 2-2 Dams in Series—Upper Dam ........................................................................................................................................................ 2-4 Dams in Series—Lower Dam ........................................................................................................................................................ 2-4 Large Drainage Areas...................................................................................................................................................................... 2-5

PART 3 – SEDIMENTATION ............................................................................................................. 3-1

PART 4 – GEOLOGIC AND GEOTECHNICAL CONSIDERATIONS .......................................................... 4-1 Soils With Dispersive Clays ........................................................................................................................................................... 4-1 Karst ...................................................................................................................................................................................................... 4-1 Collapsible Soils ................................................................................................................................................................................ 4-1 Liquefaction Susceptibility ........................................................................................................................................................... 4-2 Seismicity and Earthquake Loading .......................................................................................................................................... 4-2 Auxiliary Spillways .......................................................................................................................................................................... 4-4 Mass Movements .............................................................................................................................................................................. 4-4 Subsidence .......................................................................................................................................................................................... 4-4 Multipurpose and Water Retention Dams .............................................................................................................................. 4-4 Other ..................................................................................................................................................................................................... 4-4

PART 5 – EARTH EMBANKMENTS AND FOUNDATIONS .................................................................... 5-1 Height.................................................................................................................................................................................................... 5-1


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Top Width ............................................................................................................................................................................................ 5-1 Embankment Slope Stability ........................................................................................................................................................ 5-2 Static Stability .................................................................................................................................................................................... 5-3 Stability During Construction ...................................................................................................................................................... 5-3 Rapid Drawdown .............................................................................................................................................................................. 5-3 Steady Seepage .................................................................................................................................................................................. 5-4 Flood Surcharge ................................................................................................................................................................................ 5-4 Dynamic Stability ............................................................................................................................................................................. 5-7 Analysis ................................................................................................................................................................................................ 5-8 Sites With Limited Loss of Strength Under Earthquake Loading.................................................................................... 5-9 Sites With the Potential for Significant Loss of Strength under Earthquake Loading ............................................ 5-9 Seepage .............................................................................................................................................................................................. 5-10 Analysis .............................................................................................................................................................................................. 5-10 Material Properties ....................................................................................................................................................................... 5-11 Design Requirements.................................................................................................................................................................... 5-12 Geosynthetics ................................................................................................................................................................................... 5-14 Zoning ................................................................................................................................................................................................. 5-15 Surface Protection.......................................................................................................................................................................... 5-15 Vegetative Protection ................................................................................................................................................................... 5-15 Structural Protection .................................................................................................................................................................... 5-16

Observation and Instrumentation ........................................................................................................................................... 5-16 Parapet Walls ................................................................................................................................................................................... 5-16

PART 6 – PRINCIPAL SPILLWAYS ..................................................................................................... 6-1 Capacity of Principal Spillways ................................................................................................................................................... 6-1 Elevation of Principal Spillways ................................................................................................................................................. 6-1

Single-purpose floodwater retarding dams ............................................................................................................................................. 6-1 Other dams .............................................................................................................................................................................................................. 6-2

Routing of Principal Spillway Hydrographs ........................................................................................................................... 6-2 Design of Principal Spillways ....................................................................................................................................................... 6-2

Hydraulics ................................................................................................................................................................................................................ 6-2 Risers ......................................................................................................................................................................................................................... 6-2 Conduits .................................................................................................................................................................................................................... 6-3 Cast-in-place reinforced concrete conduits .............................................................................................................................................. 6-4 Reinforced concrete pressure pipe conduits ........................................................................................................................................... 6-4 Conduit diameter .................................................................................................................................................................................................. 6-4 Corrugated steel pipe or welded steel pipe conduits ........................................................................................................................... 6-5


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Joints .......................................................................................................................................................................................................................... 6-6 Replacement or installation of conduits in existing embankments ............................................................................................... 6-7 Conduit abandonment ....................................................................................................................................................................................... 6-8 Conduit filters ........................................................................................................................................................................................................ 6-8

Outlets .................................................................................................................................................................................................. 6-8 Trash Racks ........................................................................................................................................................................................ 6-9 Antivortex Devices ........................................................................................................................................................................... 6-9

PART 7 – AUXILIARY SPILLWAYS ..................................................................................................... 7-1 Closed-Conduit Auxiliary Spillways........................................................................................................................................... 7-1 Spillway Requirements .................................................................................................................................................................. 7-1

Capacity of auxiliary spillways ....................................................................................................................................................................... 7-1 Elevation of the crest of the auxiliary spillway ....................................................................................................................................... 7-2

Auxiliary Spillway Routings ......................................................................................................................................................... 7-2 Hydraulic Design .............................................................................................................................................................................. 7-2 Structural Stability ........................................................................................................................................................................... 7-3 Vegetated and Earth Auxiliary Spillways ................................................................................................................................ 7-3

Layout ........................................................................................................................................................................................................................ 7-4 Stability design of vegetated and earth spillways ................................................................................................................................. 7-5 Integrity design of vegetated and earth earth spillways .................................................................................................................... 7-5 Special precautions for high hazard potential dams. ........................................................................................................................... 7-5 Barriers to stop headcut progression in earth spillways ................................................................................................................... 7-6 Armored earth auxiliary spillways ............................................................................................................................................................... 7-6

In-Situ Rock Auxiliary Spillways ................................................................................................................................................. 7-7 Structural Auxiliary Spillways ..................................................................................................................................................... 7-7

GLOSSARY ...................................................................................................................................... 7-9

REFERENCES ................................................................................................................................. 7-13

FIGURES Figure 2–1 Table of Minimum Principal Spillway Hydrologic Criteria ......................................................................................... 2–3

Figure 2–2 Table of Minimum Auxiliary Spillway Hydrologic Criteria ........................................................................................ 2–4

Figure 4–1 Table of Earthquake Loading – Min Peak Ground Acceleration for Seismic Evaluation of Dams/Appurtenances ................................................................................................................................................................... 4–3

Figure 4–2 Table of Seismic Requirements for Postearthquake Operational Evaluation of Dams .................................. 4–4

Figure 5–1 Table of Minimum Top Width of Embankment ............................................................................................................... 5–2

Figure 5–2 Mohr-Coulomb Envelope for Upstream Drawdown ..................................................................................................... 5–6

Figure 5–3 Table of Slope Stability Criteria .............................................................................................................................................. 5–6

Figure 5–4 Table of Shear Strength Parameters, Associated Tests, and Nomenclature ....................................................... 5–7


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PART 1 – GENERAL Dam Classification Title 210, National Engineering Manual, Part 520, Subpart C, “Dams” (210-NEM-520-C), establishes NRCS policy on dam classification. Classification requires consideration of damage that may occur to existing and future developments should the dam suddenly release large quantities of water downstream due to a breach, failure, or landslide into the reservoir. Embankment, spillway, and reservoir characteristics; physical characteristics of the site and the valley downstream; and relationship of the site to industrial and residential areas, including controls of future development, all have a bearing on the amount of potential damage in the event of a failure.

The assigned dam classification, as determined by the above conditions influencing the potential hazard to life and property should the dam suddenly breach or fail, provides the basis for design criteria. If there is a potential for change in hazard potential classification over the design life of the structure because of future development, design of the dam should incorporate criteria appropriate for the potential future hazard for elements of the dam and appurtenances that are problematic to update. These include such elements as—

• Foundation treatment.

• Embankment zoning.

• Principal spillway material.

• Areas needed for future increase to size of dam and auxiliary spillways.

Consideration of a future change in hazard potential classification and associated design criteria may also include physical or administrative provisions to facilitate modifying dam, and auxiliary spillway dimensions, or pool elevations. Designs must also recognize State and local regulations and the responsibility of the involved public agencies.

Classes of Dams 210-NEM-520 classifies dams as follows:

• Low Hazard Potential.—Dams in rural or agricultural areas where failure may damage farm buildings, agricultural land, or township and country roads.

• Significant Hazard Potential.—Dams in predominantly rural or agricultural areas where failure may damage isolated homes, main highways or minor railroads, or interrupt service of relatively important public utilities.

• High Hazard Potential.—Dams where failure may cause loss of life or serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or railroads.


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Peak Breach Discharge Criteria Use breach routings to help delineate the area potentially impacted by inundation should a dam fail and to aid dam hazard potential classification. Develop routings using topographic data and hydraulic methodologies mutually consistent in their accuracy and commensurate with the level of risk under evaluation. For hazard potential classification, evaluate probable downstream conditions that could exist for the failure mode being evaluated, and incorporate the condition that would represent the highest hazard into routings. Federal Emergency Management Agency (FEMA) 333, “Federal Guidelines for Dam Safety: Hazard Potential Classification System for Dams,” requires the assignment of classification “based on the worst-case probable scenario of failure or misoperation of the dam,” meaning assignment of hazard potential classification “based on failure consequences that will result in the assignment of the highest hazard potential classification of all probable failure and misoperation scenarios.”

Evaluate dam failure with the water surface elevation of the reservoir at the dam crest or the peak reservoir stage resulting from the probable maximum flood (PMF). The minimum peak discharge of the breach hydrograph, regardless of the technique used to analyze the downstream inundation area, is—

1. For depth of water at the dam at the time of failure where 𝐻𝐻𝑤𝑤 ≥ 103𝑓𝑓𝑓𝑓

𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 65 𝐻𝐻𝑤𝑤1.85

2. For depth of water at the dam at the time of failure where 𝐻𝐻𝑤𝑤 < 103𝑓𝑓𝑓𝑓

𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 1100 𝐵𝐵𝑟𝑟1.35 where 𝐵𝐵𝑟𝑟 =𝑉𝑉𝑠𝑠𝐻𝐻𝑤𝑤𝐴𝐴

But not less than 𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 3.2 𝐻𝐻𝑤𝑤2.5 nor more than 𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 65 𝐻𝐻𝑤𝑤1.85

3. When the width of the valley, L, at the water surface elevation corresponding to the depth, Hw, is less than—

𝑇𝑇 =65𝐻𝐻𝑤𝑤0.35

0.416

replace the equation, 𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 65 𝐻𝐻𝑤𝑤1.85, in 1 and 2 above with—

𝑄𝑄𝑚𝑚𝑚𝑚𝑚𝑚 = 0.416𝐿𝐿𝐻𝐻𝑤𝑤1.5

Where—

Qmax = peak breach discharge, cubic feet per second

Br = breach factor, for the equation, 𝐵𝐵𝑟𝑟 =𝑉𝑉𝑠𝑠𝐻𝐻𝑤𝑤𝐴𝐴

, acre

Vs = reservoir storage at the time of failure, acre feet

Hw = depth of water at the dam at the time of failure; however, in the case of dam


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overtopping, not to exceed depth at the top of the dam, feet

A = cross-sectional area of embankment at the assumed location of breach, usually the template section (normal to the dam longitudinal axis) at the general floodplain location, square feet

T = theoretical breach width at the water surface elevation corresponding to the depth, Hw, for the equation, 𝑄𝑄max = 65 𝐻𝐻𝑤𝑤1.85,𝑓𝑓𝑓𝑓

L = width of the valley at the water surface elevation corresponding to the depth, Hw, feet

The peak discharge value determined by using principles of erosion, hydraulics, and sediment transport may be used in lieu of the peak discharge computed using the equations presented. Examples of acceptable, process-based models include the National Weather Service (NWS) BREACH model and NRCS WinDAM.

Cut Slope Stability Plan and form excavated cut slopes in a stable and safe manner. Spillways, inlet and outlet channels, borrow pits, reservoir edges, abutment areas, and foundation excavations are all locations where these considerations are needed. Field investigations, methods of analysis, design and construction requirements, and resultant specifications must recognize and provide for safe functional performance. Part 4 of this TR discusses the requirements for a geotechnical investigation plan that may include the evaluation of natural slope stability. Part 5 of this TR discusses the stability evaluation of constructed slopes.

Reservoir Conservation Storage Analyze reservoirs with water stored for conservation purposes using a water budget to determine a dependable water supply. For most purposes—

• NRCS defines a dependable water supply as one that is available at least 8 out of 10 years or has an 80-percent chance of occurring in any one year.

• A purpose such as municipal and industrial water supply may require a 95-percent chance of occurring in any one year.

• Other purposes, such as recreation, require an analysis of the reservoir surface elevation fluctuation to evaluate the acceptable percent chance of occurrence.

Joint Use of Reservoir Capacity Efficient use of a reservoir site occurs where hydrologic conditions permit joint use of storage capacity by floodwater and conservation storage. For joint-use storage dams, NRCS requires—

• Reasonable assurance of adequate water supply to meet project objectives.

• Satisfaction of flood protection objectives of the project.

• Spillway conditions that will enable the dam to perform safely.

NRCS may require special hydrologic studies to show compliance with the requirements listed above.


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This may include hydrometeorologic instrumentation and analysis.

Hydraulic features must include an ungated spillway outlet at the top of the joint use pool. At the bottom of the joint use pool, provide a gated opening or other reliable means to remove water from the conservation storage to meet project objectives.

Make provisions for operation of the joint-use pool to ensure functioning of the dam as designed. These must include a competent operating and maintaining organization and a specific operation and maintenance (O&M) plan. These requirements must be a part of the planning process and agreed to by the sponsors or owner.

Visual Resource Design The public generally prefers lake or waterscape scenery. Therefore, when dam construction creates permanent pools, emphasis on water views can enhance the visual resource. A visual design objective must focus public views toward the permanent pool and reduce the visual focal effects of the structural elements.

Achieve visual focus on the lake by locating roads and walkways so that the entering or first perceptions of the site are of the waterscape scenery. In most landscapes, the lake will automatically predominate when the visual design keeps other elements subordinate.

Shape borrow areas to blend with the surrounding topography. Vegetate these areas with herbaceous and woody plants to provide a visual fit to the existing surrounding vegetation. As much as possible, locate fences parallel to the contour, behind existing vegetation as seen from the major viewpoints, and low in the landscape. Shape dams to blend with the natural topography to the extent feasible.

Safety and Protection Many dams have features potentially hazardous to the public. Features designed for recreation or fish and wildlife are especially attractive to the public since they provide an opportunity to use the water. Design all dams to avoid hazardous conditions where possible. Open-top risers, steep-walled channels and chutes, plunge pools, and stilling basins are hazardous and require special attention. All dams must include safety fences, guardrails, or other safeguards necessary to protect the public and O&M personnel. Provide fences or other barriers where necessary to protect the dam from livestock, foot traffic, and vehicular traffic.

Water Supply Pipes Water supply pipes or conduits for other purposes installed under any part of the embankment must—

• Provide durability, strength, and flexibility equivalent to the principal spillway.

• Include features to facilitate compaction for seepage control and filtered as required for principal spillway pipes.

• Be encased throughout the footprint of the embankment.

• Be watertight against anticipated pressures.

• Be adequate for their intended use.


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• Have shutoff capability upstream and downstream of the embankment.

NRCS does not permit water supply pipes through previously constructed embankments unless there is no other technically viable alternative. Water supply pipes through previously constructed embankments must address all settlement, stability, seepage, and construction issues as other penetrations and must not damage or adversely affect any embankment zone or appurtenances.

Utility Cables and Pipelines Dam sites frequently have existing pipelines, cables, and conduits of a wide variety of sizes, materials, and functions. These conduits are usually located at shallow depths in the floodplain. They constitute a hazard to the safety of the dam and require either relocation away from the dam, reconstruction, or modification. Reconstructed conduits must provide the durability, strength, and flexibility equal in all aspects to the principal spillway designed for the site, in accordance with NRCS criteria and procedures. Relocate or raise overhead cables or power lines as necessary to prevent damage or hazard to the public.

Designers should make every reasonable effort to have such conduits, cables, and pipelines removed from the site. Most utilities and industries will want their facility removed from the site for easy maintenance. NRCS only permits conduits to remain under an earth dam embankment as a last resort and under the limitations imposed.

Conduits remaining under any part of the embankment must have—

• Features to facilitate compaction for seepage control and filters as required for principal spillway pipes.

• Proper articulation on all yielding foundations.

• Encasement by concrete or otherwise treated to ensure durability and strength equal to that of the principal spillway.

• Watertight features preventing leakage either into or out of the casing and carrier pipe.

Acceptable enclosure of the conduit cable or pipeline requires enclosing the conduit within another conduit that meets the requirements of this section and positively sealing the upstream end to prevent seepage into the enclosing conduit. Such an enclosing conduit must extend the full distance through which the enclosed conduit, cable, or pipeline is beneath the embankment.

NRCS does not permit utilities through previously constructed embankments unless there is no other technically viable alternative. Penetrations for utilities through previously constructed embankments must address all settlement, stability, seepage, and construction issues as other penetrations and must not damage or adversely affect any embankment zone or appurtenances.

Streamflow Diversion during Construction Determine requirements for diversion and care of streamflow during construction in the planning and design process and include diversion requirements in the specifications.

The construction of the permanent work and diversion features, such as cofferdams and ancillary structures, create a construction hazard from the time that structures obstruct natural streamflow until


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the dam and appurtenances are complete and are operational. The damage from overtopping or exceeding the capacity of the diversion works during construction can range from erosion repair and cleanup costs to a loss of the temporary diversion works, complete breach of the dam, or damage to the appurtenances.

Evaluate hazards during construction, including the costs of damage to the partially completed work, delay of function of the completed structure from extension of construction time, downstream hazard during different phases of construction, and environmental consequences, such as sedimentation from erosion of the diversion or permanent work.

During construction, a greater risk usually exists than the risk that exists after the dam is completed. The risk of damage or overtopping depends on—

• Hydrology.

• Watershed size.

• Upstream structures in the watershed.

• Construction duration.

• Individual project characteristics including—

o New or rehabilitation construction. o Type of construction. o Embankment materials. o Site conditions.

• Diversion capacity.

Develop a suite of inflow floods spanning return periods of interest for evaluation of diversion requirements. Identify seasonal variability that may affect diversion requirements or construction schedule if applicable. Evaluate stage-duration-frequency information for a range of diversion conditions.

Select the tolerable level of construction risk and the diversion requirements to achieve a lower level of construction risk, based on the identified construction hazards. The allowable probability of inflow exceeding diversion capacity varies from a 20-percent chance in any 1 year for sites with limited hazard to less than a 1-percent chance in any 1 year for sites where there is extensive downstream hazard or there would be significant repair cost to the work.

Size the diversion to provide an acceptable level of risk during the construction period. As appropriate, specify cofferdam and diversion conveyance requirements, erosion protection, construction sequencing, timeframes and milestones required for diversion.


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PART 2 – HYDROLOGY Introduction This part describes hydrologic criteria for determining spillway discharges and floodwater storage volumes. Title 210, National Engineering Handbook, Part 630, Chapter 21, “Design Hydrographs” (210-NEH-630-21), provides a detailed description of acceptable procedures and example problems for development of principal spillway, auxiliary spillway, and freeboard design hydrographs for TR 210-60, “Earth Dams and Reservoirs.” 210-NEH-630-17, “Flood Routing,” contains methods of flood routing hydrographs through reservoirs and spillway systems.

Special studies, as used in this text, refer to all site-specific studies undertaken with prior concurrence of the Director, Conservation Engineering Division.

Obtain precipitation data from the most recent National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) reference applicable to the area under study or from other special studies as appropriate. 210-NEH-630-21 contains a partial listing of references.

When computing runoff volumes using the NRCS runoff curve number procedure defined in 210-NEH-630-9, “Hydrologic Soil-Cover Complexes,” and 210-NEH-630-10, “Estimation of Direct Runoff from Storm Rainfall,” assume an antecedent runoff condition (ARC) II or greater for obtaining the runoff curve number. Apply the same runoff curve number throughout the entire storm.

Adjustments to runoff curve numbers and runoff volumes may be applicable in development of the design hydrographs. 210-NEH-630-21 includes examples of these optional adjustments. Make detailed evaluations to determine the appropriateness of these optional adjustments.

Principal Spillway Design Hydrographs Use the runoff from a storm duration of not less than 10 days for sizing the principal spillway. The return period for design precipitation amounts depends on the dam’s hazard potential classification, purpose, size, location, and type of auxiliary spillway. Figure 2–1 below shows minimum return period principal spillway hydrologic criteria by hazard potential classification.


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Figure 2–1: Table of Minimum Principal Spillway Hydrologic Criteria

1. Precipitation amounts by return period in years. In some areas, NRCS allows the use of direct runoff amounts determined using procedures in 210-NEH-630, “Hydrology,” in lieu of precipitation data.

2. Applies to irrigation dams on ephemeral streams in areas where the annual rainfall is less than 25 inches. 3. Increase the minimum earth spillway requirements to P100 for a ramp spillway. 4. Design low hazard potential dams involving industrial or municipal water with a minimum criteria equivalent to that of significant hazard potential. 5. Applies when the upstream dam is located so that its failure could endanger the lower dam.

Select the procedure for estimating runoff volumes that requires the higher auxiliary spillway crest elevation when routing the principal spillway hydrograph through the structure. 210-NEH-630-21 describes procedures used to estimate runoff volumes. The procedure for developing the 10-day principal spillway mass curve described in 210-NEH-630-21 uses both the 1-day and 10-day runoff volumes and includes procedures for making areal adjustments if appropriate.

In arid and semiarid climatic areas when the climatic index, as defined in 210-NEH-630-21, is less than one, NRCS allows the use of transmission losses to reduce the runoff. NRCS requires special studies when transmission losses appear to be significant, such as in cavernous areas, even though the climatic index is one or more.

Use procedures described in 210-NEH-630-16 and 630-21, and applicable national computer programs to develop the principal spillway hydrograph.

NRCS allows the use of streamflow records to develop the principal spillway hydrograph where a special study shows the adequacy of this procedure for this purpose.

Auxiliary Spillway and Freeboard Hydrographs Figure 2–2 below, shows minimum auxiliary spillway hydrologic criteria by hazard potential classification.

Evaluate the discharge capacity, stability (surface erosion potential), and integrity (breaching

Class of dam

Purpose of dam

Product of storage (AC-FT) × effective

height (FT)

Existing or planned upstream dams

Precipitation data for maximum frequency of use of auxiliary spillway types1

Earth Vegetated

Low hazard

Single irrigation

only2

less than 30,000 none ½ design life ½ design life

greater than 30,000 none ¾ design life ¾ design life

Single or multiple4

less than 30,000 none P50 P253

greater than 30,000 none P75 P503

all Any5 P100 P503

Significant hazard

Single or multiple all none or any P100 P50

High hazard Single or multiple all none or any P100 P100


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potential) of the auxiliary as follows:

• Analyze both a 6- and 24-hour duration storm. If the drainage area is large and the time of concentration exceeds 24 hours, analyze longer duration storms to ensure evaluation of all significant storm events. Use the most critical results to check the discharge capacity and the integrity of the auxiliary spillway.

• For locations where NWS references provide estimates of local storm, general storm values, and other types of storm values, analyze all.

• Use the short-duration storm to check the stability of vegetated and earth auxiliary spillways.

• In areas without NWS references for spatial distribution, NRCS allows the use of minimum areal adjustment ratios as described in 210-NEH-630-21 for drainage areas greater than 10 square miles.

• In areas without NWS reference for temporal distribution, NRCS allows the use of the dimensionless auxiliary and freeboard storm distribution as described in 210-NEH-630-21, “Design Hydrographs.” Alternately, develop the NRCS 24-hour five-point storm distribution by critically stacking incremental rainfall amounts of successive 6-, 12-, and 24-hour durations as described in NOAA Hydrometerological Report 52, “Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian.”

NRCS allows consideration of special probable maximum precipitation (PMP) studies. NWS or other hydrometeorologists with experience in such work may conduct such studies. Consider special studies for large drainage areas, areas of significant variation in elevation, or areas located at the boundary of two studies where discontinuities occur. Compare the results of the special study with the base NRCS PMP and provide justification of using the special study results if less than the base NRCS PMP.

Use procedures in 210-NEH-630-16 and 630-21, and applicable national computer programs to develop stability design (auxiliary spillway) and freeboard hydrographs. NRCS allows methods outlined in FEMA 94, “Selecting and Accommodating Inflow Design Floods for Dams,” as an alternate method to determine the freeboard hydrograph, provided downstream land use controls exist to prevent the voiding of incremental risk assumptions after dam construction.

Figure 2–2: Table of Minimum Auxiliary Spillway Hydrologic Criteria

Class of Dam Product of storage

(AC-FT) × effective height (FT

Existing or planned upstream dams

Precipitation data for1

Auxiliary spillway hydrograph Freeboard hydrograph

Low hazard2

less than 30,000 none

P100 P100+0.12(PMP–P100)

greater than 30,000 P100+0.06(PMP–P100) P100+0.26(PMP–P100)

All any3 P100+0.12(PMP–P100) P100+0.40(PMP–P100)

Significant hazard All none or any P100+0.12(PMP–P100) P100+0.40(PMP–P100)

High hazard All none or any P100+0.26(PMP–P100) PMP


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1. P100, = Precipitation for 100-year return period; PMP = Probable maximum precipitation. 2. For dams involving industrial or municipal water, use minimum criteria equivalent to that of a significant hazard potential dam. 3. Applies when the upstream dam is located so that its failure could endanger the lower dam.

Dams in Series—Upper Dam In a system of dams in series, if failure of the upper dam could contribute to failure of the lower dam, the hydrologic criteria and procedures for the design of the upper dam must be the same as, or more conservative than, those for the dam downstream.

Dams in Series—Lower Dam In design of the lower dam, use the time of concentration (Tc) of the watershed above the upper dam to develop the hydrographs for the upper dam. Use the Tc of the uncontrolled area above the lower site to develop the uncontrolled area hydrographs. If the Tc for the total area exceeds the storm duration, increase the precipitation amounts for the auxiliary spillway and freeboard hydrographs by the values in NWS references.

In designing the lower dam, take the following steps:

Step 1—Using the hydrologic design criteria for the lower dam, develop the runoff hydrograph for the area controlled by the upper dam.

Step 2—Route the inflow hydrograph for the upper dam through the upper dam. If the upper dam is overtopped or its safety is questionable, it is considered breached or failed and the breach hydrograph from the upper dams is used as the outflow hydrograph for the design of the lower dam.

Step 3—Route the outflow hydrograph for the upper dam to the lower dam.

Step 4—Develop the runoff hydrograph for the intermediate uncontrolled drainage area of the lower (downstream) dam.

Step 5—Combine the routed outflow hydrograph from the upper dam with the hydrograph from the intermediate uncontrolled drainage area.

Use the combined hydrograph based on the principal spillway hydrologic criteria for the lower dam to determine the capacity of the principal spillway and the floodwater retarding storage requirement of the lower dam. Use the combined hydrograph based on the stability design (auxiliary spillway) hydrologic criteria for the lower dam to evaluate the stability (erosion resistance) of any vegetated or earth spillway at the lower site. Use the combined hydrograph based on the freeboard hydrologic criteria for the lower dam to determine top of dam and to evaluate the integrity of any vegetated or earth spillway at the lower dam.

NRCS allows the use of areal reduction factors, determined based on the total drainage area of the dam system, to reduce the minimum precipitation amounts for each of the required hydrographs.

Design of the lower dam should consider the effects of the breach hydrograph from the upper dam routed downstream to the lower dam, combined with the uncontrolled area hydrograph. To mitigate for a potential breach of the upper dam, designers of the lower dam should consider measures such as increased storage, increased spillway capacity, and overtopping protection.


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If upon routing a hydrograph through the upper dam, the dam overtops without overtopping protection, or its safety is questionable, NRCS considers the dam highly vulnerable to a breach. NRCS requires increased storage and spillway capacity, without overtopping, for a lower dam below an upper dam highly vulnerable to breach.

Design of the lower dam should consider the potential for the removal of an upper dam or dams during the service life of the lower dam. Removal of an upper dam or dams may increase lower dam design flows. To mitigate for increased design flows associated with the removal of an upper dam, designers of the lower dam should consider measures such as increased storage, increased spillway capacity, or overtopping protection. Develop lower dam designs to accommodate upper dam removal as part of initial dam construction, dam rehabilitation, or as a future enhancement prior to removal of the of the upper dam.

Large Drainage Areas When the area above a proposed dam approaches 50 square miles, it is desirable to divide the area into hydrologically homogeneous subbasins for developing the design hydrographs. Generally, the drainage area for a subbasin should not exceed 20 square miles. Use watershed modeling computer programs, such as WinTR–20 (the computer program for project formulation hydrology) or SITES (Water Resources Site Analysis Program) for inflow hydrograph development.

If the Tc for the entire drainage area is greater than 24 hours, test storm durations longer than the Tc to determine the duration that gives the maximum reservoir stage for the routed stability design (auxiliary spillway) and freeboard hydrographs.

Precipitation amounts may vary markedly within a large watershed due to topographical and meteorological parameters such as aspect, orientation, mean elevation of subbasin, and storm orientation. Consider the use of a special PMP study for large watersheds with drainage areas more than 100 square miles. Individual watershed PMP studies can consider orographic features typically smoothed in the generalized precipitation studies. Areas where significant snowmelt can occur during the design storms may warrant special studies.

NRCS encourages unit hydrograph development using watershed storage and timing effects, watershed model calibration, and similar studies using watershed stream flow records.


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PART 3 – SEDIMENTATION Reservoirs used to store or retard water from surface runoff will trap and store a large portion of the sediment in the runoff water. Sediment accumulation in the reservoir can adversely influence operation of the low flow gates and principal spillway as well as reduce conservation and flood storage. Therefore, allocate storage capacity for the calculated sediment accumulation during the design life of the reservoir. Designers should also consider measures to address passing sediment through the dam, capturing sediment in the sediment basin, removal of sediment, or providing storage within the reservoir for sediment.

210-NEH-3, “Sedimentation” contains criteria and general procedures needed to determine the volume required for sediment accumulation and its allocation in the reservoir. It also includes procedures for determining—

• Sediment yield for present conditions and for the future after planned land treatment and the application of other measures in the drainage area of the dam.

• Trap efficiency of the reservoir.

• Distribution and types of sediment expected to accumulate.

• Proportion of continuously submerged sediment versus aerated sediment.

• Densities to which the sediment will become compacted.

If the amount of sediment accumulation calculated exceeds 2 watershed inches in 50 years for the uncontrolled drainage area of the dam, reevaluate the entire watershed to determine the feasibility and applicability of more economical methods of reducing sediment yield or trapping sediment.

When rehabilitating existing dams, use guidance in 210-NEH-3 to perform a sedimentation analysis. In addition, collect the following information on the existing structure:

• Sediment deposition amount and distribution

• Density of deposited sediment

• Proportion of continuously submerged sediment versus aerated sediment.

• Remaining sediment storage available

• Impacts on dam operation

• Projected future sediment deposition


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PART 4 – GEOLOGIC AND GEOTECHNICAL CONSIDERATIONS Investigate site geologic and geotechnical conditions in a manner that adequately examines embankments, spillways, abutments, borrow areas and foundations to enable adequate evaluation of all design conditions. Provide appropriate intensity and detail of these investigations for the class of dam, complexity of site geology, and the data needed for the dam design.

The design engineer, acting as team leader, geotechnical engineer, geologist, and other disciplines requiring geotechnical data, must jointly complete a geotechnical investigation plan prior to initiating geologic investigations. This plan must provide the basis for geologic investigations, and must include the sampling and testing required for—

• Addressing site geologic conditions.

• Specific requirements of the anticipated design at the stage of the subject investigation.

• General requirements of this TR.

Keep an updated testing program in alignment with the geotechnical investigation plan and provide the testing program with samples submitted for testing. Complete a draft report of geologic investigation and provide for use prior to preparing the soil mechanics report. At a minimum, the draft must include a description of key geotechnical and geologic issues at the site, the anticipated design, preliminary profiles and cross sections, and drill holes in stick figure format with field classifications and in-situ test results. Complete the final report of geologic investigation, including field and laboratory classifications, in final format for use in preparing the design.

210-NEM-531, “Geology,” and 210-NEH-631, “Geology,” establish the general requirements, procedures, and criteria for geologic investigations.

Site conditions and dam features that require special attention include, but are not limited to, the following geologic considerations.

Soils with Dispersive Clays Soils containing dispersive clays are extremely erosive. Dams constructed using dispersive clays are vulnerable to internal erosion failures and require special design considerations. Geologic investigations for soils containing dispersive clays require special sampling procedures. This includes obtaining many discrete samples and preserving the samples at their natural water content.

Karst Karst terrain requires detailed evaluation of potential subsidence, seepage, and leakage in the dam foundation and reservoir floor. Thoroughly evaluate these issues as they have significant impact on the design, construction, cost, performance, and safety of the structure. Multipurpose structures with permanent water storage in karst areas are especially critical. The geotechnical investigation must determine, and the report clearly state, if there are any foundation materials with dissolution potential.

Collapsible Soils Evaluate the potential of moisture deficient, low density, unconsolidated materials to collapse on saturation or wetting. NRCS typically encounters collapsible soils in arid and semiarid areas.


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Collapsible materials are often associated with deposits, such as alluvial fans, terraces, and aeolian soils. If the potential for collapsible soils exists, perform extensive site investigations and testing to provide quantitative information for design and construction. Obtain and test undisturbed samples that are representative of the collapsible material.

Liquefaction Susceptibility Soil liquefaction typically occurs in recent deposits of loose sand and silty sand located below the water table; however, gravels and low plasticity silts may also liquefy. Assess groundwater conditions and the occurrence and extent of potentially liquefiable soils that could lose strength under the earthquake shaking considered at a site. Characterize other soils that could undergo loss of strength resulting from earthquake shaking, including sensitive clays, clays and plastic silts potentially susceptible to cyclic softening, and collapsible weakly cemented soils. When developing a subsurface investigation plan, the geologist and geotechnical engineer must consider identifying and locating all layers of potentially liquefiable soils to provide data adequate to analyze the potential significant loss of strength under earthquake loading.

Seismicity and Earthquake Loading Consider the effects of earthquake loading for all dams. The level of investigation and analysis performed for site characterization and delineation of potential earthquake loading will depend on the potential consequences of seismic dam failure, the seismicity of the site, individual site characteristics that influence the performance of the dam under earthquake loading and the anticipated analysis and design methods.

Geologic investigations must document the existence or absence of active and capable faults at the site. Geologic investigation reports must include a map showing all magnitude 4 or intensity V (near epicenter), or greater earthquakes of record and any active or capable faults within a 100-kilometer (62 mile) radius of the site. Use moment magnitude (Mw) to define earthquake magnitude occurring after the 1970s.

Assess the existence and extent of potentially liquefiable soils and groundwater conditions at sites where the considered earthquake ground motions could cause liquefaction in susceptible materials.

As a minimum, use the peak ground acceleration (PGA) resulting from the maximum design earthquake loading (MDE) corresponding to the annual exceedance probability listed in figure 4–1 below. Obtain PGA and other earthquake characteristics associated with the exceedance probabilities listed in figure 4–1 from the U.S. Geological Survey (USGS), Earthquake Hazards Program. Ground motion values must be representative of the site foundation conditions. If foundation conditions representative of the site are not directly available from the USGS data, adjust the PGA values to represent the actual site conditions.

Figure 4–1: Table of Earthquake Loading – Minimum Peak Ground Acceleration for Seismic Evaluation of Dams and Appurtenances


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Consequence of a seismic failure

Exceedance probability for peak ground acceleration

(PGA)

Return period (years)

Approximate probability of

exceedance in 50 years

Low consequence 1 x 10–3 1,000 5% Significant consequence 4 x 10–4 2,500 2%

High consequence 1 x 10–4 10,000 0.5%

Consequences from seismic failure based on a failure mode with the reservoir at the normal pool elevation are—

• Low Consequence.—Dams in rural or agricultural areas where seismic failure of the dam or appurtenance may result in damage to farm buildings, agricultural land, or township and country roads.

• Significant Consequence.—Dams in predominantly rural or agricultural areas where seismic failure of the dam or appurtenance may result in damage to isolated homes, main highways or minor railroads, and interrupt service of relatively important public utilities. For high hazard potential and significant hazard potential dams, significant consequence of a seismic failure of the dam or appurtenance may also include the loss of stored water from water supply dams where there is no other water supply source. For high hazard potential and significant hazard potential dams, significant consequence also includes the loss of the permanent pool where a seismic failure of the dam or appurtenance would have significant consequences such as the loss of important recreational facilities or environment damage.

• High Consequence.—Dams where seismic failure of the dam or appurtenance may cause loss of life or serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or railroads.

Designs may use a site-specific seismotectonic study conducted in accordance with FEMA 65, “Earthquake Analysis and Design of Dams,” for significant and high hazard potential dams in lieu of using the loadings shown in figure 4–1. If earthquake parameters for high hazard potential dams are determined from a deterministic seismic hazard assessment, base parameters on mean plus one standard deviation results. Documentation of site-specific study results will include comparison of the proposed earthquake loading resulting from using the most recent version of the USGS Interactive Deaggregation Program data at the annual exceedance probability listed in figure 4–1 and the rationale for selection.

Design requirements and the methods used to evaluate earthquake effects on the dam may require the use of additional earthquake parameters to evaluate earthquake effects on the dam. Additional parameters may include magnitude and distance of features contributing most of the seismic risk, recommended design earthquakes magnitude and distance, acceleration time histories, spectral response, amplification factors, or other parameters. Document all methods and assumptions incorporated into the development of the earthquake loading and details that are required to apply the results appropriately in the dam analysis.


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As a minimum, use the peak ground acceleration (PGA) resulting from the operating basis earthquake (OBE) loading corresponding to the annual exceedance probability listed in figure 4–2 for post-earthquake operational evaluation of dams. Part 5 describes the requirements for this evaluation.

Figure 4–2: Table of Seismic Requirements for Postearthquake Operational Evaluation of Dams

Class of Dam PGA annual exceedance probability

Return period (years)

Approximate probability of

exceedance in 50 years Low hazard potential ---- ---- ----

Significant hazard potential 4 x 10–3

250 20%

High hazard potential 2 x 10–3

500 10%

Evaluate the potential for earthquake-generated seiche wave to affect the dam or appurtenances.

Auxiliary Spillways Evaluate all earth materials beneath a vegetated or earth auxiliary spillway down to the elevation of the downstream floodplain or valley floor in enough detail to determine excavation requirements, suitability of spillway excavation for embankment fill and to determine stability and integrity of the spillway. Highly erosive material, such as dispersive clays, silts and sands, and material that will require special processing for use as fill, require investigation that is more detailed. 210-NEH-628-52, “Field Procedures Guide for the Headcut Erodibility Index,” includes guidance on evaluation of headcut erodibility.

Mass Movements Evaluate landslides and landslide potential at dam and reservoir sites, especially those involving shale formations, clay layers, and those where unfavorable dip-slope or other adverse rock attitudes occur. Summarize the history of mass movement in the project area. Carefully evaluate the static and seismic stability of dam abutments, auxiliary spillway cuts and the reservoir rim. If sudden movements are possible, evaluate the effect of the potential landslide on displacement of reservoir water and resulting wave effects.

Subsidence Consult the State geological survey and data sources to investigate the potential for surface subsidence due to past or future solid, liquid (including groundwater) or gaseous mineral extraction. 210-NEM-531 provides specific requirements.

Multipurpose and Water Retention Dams Investigate and evaluate the groundwater regime and hydraulic characteristics of the entire reservoir area of water storage for potential leakage. Develop and analyze water budgets to assure the adequacy of the site to accomplish the intent of the project.

Other Special studies and evaluations may be necessary where such conditions as highly fractured bedrock


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(such as basalt or rhyolites); compacted shales; some types of siliceous, calcareous, or pyritic shales; expansive soils; soluble salts; vertic soils; mirabilite; rebound or stress relief fractures (that may cause open fissures); sharp elevation changes in foundation materials; or artesian waters occur at a site.


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PART 5 – EARTH EMBANKMENTS AND FOUNDATIONS The earth embankment and its foundation must be stable and withstand all anticipated loads without movements leading to failure. Provide measures for adequate seepage control under all anticipated loads.

Height Set the crest of an earth embankment at an elevation adequate to prevent overtopping during passage of the freeboard hydrograph. Evaluate the potential need for additional freeboard using the larger of the freeboard required for wave action or the freeboard required for frost conditions. The dam crest elevation must be above the higher of either auxiliary spillway hydrograph plus wave action or auxiliary spillway hydrograph plus frost conditions. Document the possible effects of wave action and frost conditions on the safe operation of the dam.

The dam crest evaluation must also allow for the minimum auxiliary spillway depth as described in part 7. Increase the dam crest elevation by the amount needed to compensate for settlement.

Top Width Figure 5–1 shows the minimum embankment top width.

Increase the top width from the above minimums to—

• Meet State and local standards.

• Accommodate embankment zoning.

• Provide roadway access and traffic safety.

• Provide structural stability.

When raising an embankment for rehabilitation of previously constructed embankment dam, the top width may be no more than 10 percent less than listed in figure 5–1 if analysis and design for seepage, stability, constructability, access for maintenance, and long-term reliability justifies a narrower width.

When embankment top serves as a public roadway, use the minimum width of 16 feet for one-way traffic and 26 feet for two-way traffic. Use guardrails or other safety measures meeting the requirements of the responsible road authority.

Figure 5–1: Table of Minimum Top Width of Embankment

Overall height of embankment, H, (ft)

Minimum top width (ft)

All dams Single-purpose floodwater retarding Multipurpose or other

purposes


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Overall height of embankment, H, (ft)

Minimum top width (ft)

All dams Single-purpose floodwater retarding Multipurpose or other

purposes 35


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could progress and affect deeper embankment zones, use the higher safety factors listed in figure 5–3. If cohesionless zones occur with other soil types in analysis of a cross section, use other methods to analyze circular arc or wedge-shaped surfaces that intersect the soil zones with cohesion to locate the minimum safety factor.

• Use residual effective stress parameters for modeling slope stability analyses involving fissured clays or shales if preexisting movements have occurred. Drained shear strength tests provide the basis for these parameters. Use residual parameters for designing against shallow slope failures in desiccated clay embankments with previous movement. Designers can use fully softened shear strength with conservative pore pressure assumptions to evaluate the potential for first time shallow slope failures in compacted high plasticity clay embankments.

Static Stability Analyze embankment stability for each of the following conditions in the structure life that are appropriate to the site. Clearly document the reasons for not analyzing a condition. Document the source of all shear strength parameters used. When using a shear strength parameter determined from an empirical correlation, include correlations to field performance and document the justification for use of the empirically based value rather than site-specific sampling and testing. Include discussion on the sensitivity of the outcome of the analysis to the variability of the parameter. Stability analyses must identify and incorporate any lower strength materials that may control stability.

Stability During Construction The construction sequence may control the configuration of the embankment and rate of construction when analysis predicts that embankment soil, foundation soils, or both will develop significant pore pressures during embankment construction. Evaluate site conditions, such as soft clay layers in the foundation that may require special designs. Consider flattened slopes and sequentially staged construction to achieve stable conditions. Factors determining the likelihood of this occurring include the overall height of the planned embankment, the speed of construction, the saturated consistency of foundation soils, and others. Perform appropriate shear tests to model placement conditions of embankment soils, as summarized in figure 5–3. Consider the highest likely placement water content of embankment soils in the shear-testing program. Alternatively, designers can use effective stress shear parameters to establish limits for pore pressure buildup during construction. The design should include procedures for monitoring pore pressures during construction to assure that actual pore pressures do not exceed limits established during design.

Construction stability analysis must include stability of excavation slopes that could have significant safety or construction consequences, such as excavation of existing embankment slopes during rehabilitation construction.

Rapid Drawdown Analyze the stability of the upstream embankment slope for the condition created by a rapid drawdown of the water level in the reservoir from the normal full reservoir level from which a phreatic line is likely to develop to the elevation of the lowest gated outlet. If the potential exists for saturation of zones in the upstream slope during temporary flood retention storage above the permanent normal pool, consider the effect of drawdown back to the normal pool on that saturated portion of the embankment. Designers may use transient seepage analysis to estimate the short-term saturation of the portion of the embankment above permanent normal pool. Select shear strength parameters for the


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analyses according to figure 5–3, as illustrated in figure 5–2.

Figure 5–2: Mohr-Coulomb Envelope for Upstream Drawdown

In cases where rapid drawdown slope stability is critical to embankment geometry or zoning, or for modifications to a previously constructed embankment, designers can also analyze rapid drawdown using procedures included in U.S. Army Corps of Engineers (USACE) Engineer Manual (EM) 1110-2-1902, Appendix G. Documentation should include the difference in analysis results between the methods and justification to use the selected results.

Steady Seepage Analyze downstream slope stability of new or previously constructed dams under steady-state seepage conditions. Perform the analysis with the reservoir elevation at the highest normal pool level. Use shear parameters as listed in figure 5–3. Base the location of the steady-state phreatic surface on the highest normal reservoir pool elevation. Designers can develop the phreatic surface for the analyses using computer programs for seepage analysis, flow nets, or Casagrande procedures.

Existing embankments may not have achieved steady-state conditions. Thus, use the calculated long-term high-level steady-state seepage condition for both new and existing embankments. For existing embankments, compare the computed phreatic surface with field performance and field measurements, when available, to assure that the computed phreatic surface meets or exceeds that which has occurred over the life of the structure.

Include a separate foundation phreatic surface as appropriate at sites with limited foundation seepage cutoff, particularly in sites with confined seepage that results in uplift at the downstream toe.

If steady-state seepage stability is highly dependent on the success of internal drainage features to control the phreatic surface, check stability assuming partially functioning or plugged drainage systems.

For appropriate slopes, designers can use infinite slope equations and applicable safety factors listed in figure 5-3.

Flood Surcharge Analyze downstream slope stability of new or previously constructed dams under flood detention conditions. Perform the analysis with the reservoir elevation at the freeboard hydrograph pool level. Use shear parameters as listed in figure 5–3.


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Develop a phreatic surface resulting from the maximum reservoir elevation. Consider material properties and the potential for defects in the upper portion of the embankment when selecting the appropriate seepage analysis and evaluating seepage conditions that could develop. Also consider the potential for increase in pore pressures in the normally saturated portion of the foundation or embankment that may result from the higher reservoir loading. Evaluate a range of potential seepage conditions to determine the sensitivity of flood storage stability to potential seepage conditions.

If short term flood storage stability is highly dependent on the success of internal drainage features to control the phreatic surface, check stability assuming partially functioning or plugged drainage systems.

For appropriate slopes, designers can use infinite slope equations and applicable safety factors listed in figure 5-3.


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Figure 5–3: Table of Static Slope Stability Criteria

Design condition

Primary assumption Remarks

Applicable shear strength parameters

Minimum safety factor

1.

Construction Stability

(upstream or downstream

slope)

Zones of the embankment or

layers of the foundation expected

to develop significant pore pressures during

construction

Low-permeability embankment soils should be tested at water contents that are as wet as likely during construction (usually wet of optimum). Saturated low permeable foundation soils not

expected to consolidate fully during construction.

Existing dams with additional fill placed above saturated low-permeability zones.

Unconsolidated; Total stress consistent with preconstruction stress

state

1.4 for failure surfaces

extending into foundation

layers 1.3 for

embankments on stronger foundations where the

failure surface is located

entirely in the embankment

Embankment zones and/or strata not

expected to develop significant pore pressures during

construction

Embankment zones, foundation strata, or both comprised of material with a permeability high enough to drain as rapidly as they are loaded

Effective stress

2.

Rapid drawdown (upstream

slope)

Drawdown from the highest normal pool to the lowest gated

outlet

Consider failure surfaces both within the embankment and extending into the foundation

Low-permeability embankment and foundation

soils that will have limited drainage during reservoir drawdown

Embankment zones, foundation strata, or both comprised of material with a permeability high enough to drain as the reservoir is drawn down

Lowest of effective stress or consolidated; total stress; consistent

with predrawdown consolidation stresses

(See fig. 5-1)

Effective stress

1.2

1.1 for near surface (infinite

slope) failure surfaces in

cohesionless soils

3.

Steady seepage

Reservoir water surface at highest

normal pool. Phreatic surface

developed from the highest normal pool;

typically the principal spillway

crest

Consider failure surfaces within both the embankment and extending into the foundation.

Foundation analysis may require separate phreatic surface evaluation, particularly in sites

with confined seepage that results in uplift at the downstream toe.

Effective stress 1.5



cohesionless soils

4.

Flood surcharge

Reservoir at freeboard

hydrograph level. Steady seepage phreatic surface incorporating

increased pore water pressure that may occur from flood

detention and pore water pressure from short term seepage

resulting from reservoir surface above the normal

pool elevation

Consider failure surfaces within both the embankment and extending into the foundation.

Embankment zones, foundation strata, or both comprised of material with a permeability high

enough to drain rapidly with changes in reservoir elevation

Low-permeability embankment and foundation

soils that will have limited drainage as the increased reservoir load is applied

Effective stress

Lowest of effective stress or consolidated;

total stress (See fig. 5-1)

1.4



cohesionless soils


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Figure 5–4: Shear Strength Parameters, Associated Tests, and Nomenclature

Shear strength parameter Types of shear tests

NRCS notation

Other common abbreviation Notes

American Standard for

Testing Materials

(ASTM) test standard

Total stress; unconsolidated1

Unconsolidated undrained triaxial UU Q

D2850

Unconfined-compression qu

Suitable for limited types of soils2

D2166

Field vane shear

Limited to saturated low strength (Su


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Limit embankment and foundation deformation resulting from the design earthquake to an amount that will not result in embankment overtopping from crest settlement, internal erosion from cracking or damage to appurtenances to the extent that could result in dam failure. Dam failure is instability or major damage that may lead to uncontrolled loss of the reservoir when subjected to the design earthquake loading.

Limit damage from the operating basis earthquake to the extent that, after the earthquake and prior to repair, the dam and appurtenances will pass the principal spillway flood without failure.

Design appurtenant structures such that earthquake-caused damage to structures will not lead to dam failure. Evaluation of appurtenances must consider damage that could result from embankment or foundation permanent deformation, in addition to transient seismic forces.

Evaluate the potential for fault movement in the dam foundation and the stability of the reservoir rim.

Do not locate new dams on active faults. Do not locate new significant or high hazard potential dams on capable faults, without specific design features that address potential fault movement.

Existing dams located on active faults must be able to withstand the potential fault offset that could result from the design earthquake, without failure.

Existing dams with high consequence from a seismic failure, located on capable faults, must withstand the potential fault offset resulting from the design earthquake, without failure.

Existing dams located on active faults must withstand the potential fault offset resulting from the operating basis earthquake and must, prior to repair following an earthquake, pass the principal spillway flood without failure.

Design and construct or rehabilitate embankment dams to resist earthquake loading with sound defensive measures representative of the current practice and commensurate with the seismicity of the site, site geology, and hazard of the structure.

Analysis Seismic analyses should begin with the simplest and most conservative method to analyze the embankment and foundation. If these analyses show that the structure will perform adequately under the design earthquakes, then further analyses are unnecessary. If further analyses are needed, the analyses must be progressively more detailed and complex. Provide a determination of earthquake loading, site characterization and determination of material properties consistent and appropriate for the level of detail and complexity of the analyses.

Sites where the design peak horizontal ground acceleration (PGA) determined in accordance with part 4 of this TR is equal to or less than 0.07 g require no seismic analysis. This low level of earthquake loading should not significantly distress embankment dams, even with site conditions susceptible to damage from earthquake loading.

If the design ground motion exceeds 0.07 g, evaluate the potential for loss of shear strength due to liquefaction or cyclic failure under seismic loading.

For seismic analyses, assume that the reservoir is at the highest normal pool elevation. Base the extent


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of saturation of embankment and foundation materials on the steady-state seepage conditions prior to earthquake loading resulting from the same pool elevation. Initial embankment and foundation properties used in the analyses must represent existing conditions prior to earthquake loading. Consider both upstream and downstream failure.

The characteristics and extent of soils that could undergo an extensive loss of strength or liquefy in the foundation and embankment must be determined. Determine the characteristics and extent of sensitive clays, clays, and plastic silts that, under the considered earthquake loading, may be susceptible to cyclic softening, and collapsible weakly cemented soils at the site.

If these materials are or may be present at the site, characterize and analyze the site as a site with the potential for significant loss of strength under earthquake loading.

If these materials are not present, characterize and analyze the site as a site with limited loss of strength under earthquake loading.

Sites With Limited Loss of Strength Under Earthquake Loading Well-built embankment dams on rock or dense soil—particularly clay—foundations generally perform well under earthquake loading. Well-built dams consist of well-compacted earth or earth and rock fill, with adequate static factors of safety, seepage control and freeboard, constructed under controlled conditions to ensure achievement of the design intent. For guidance, see Federal Emergency Management Agency (FEMA) 65, “Federal Guidelines for Dam Safety: Earthquake Analyses and Design of Dams.”

Embankments possessing these favorable characteristics, where the peak horizontal acceleration at the base of the embankment from the design earthquake is 0.20 g or less, do not require special seismic analysis.

Dams that do not have these characteristics, or where the design earthquake loading is higher, require additional analyses. Analyses should progressively increase in scope and complexity, based on data commensurate with the level of analysis. The analyses must proceed to the extent required to determine conclusively that the dam would not fail as defined above under earthquake loading. The analysis should document that the expected performance for new and existing embankments is adequate or that existing dams can achieve adequate performance by incorporating remedial measures. The analyses must include assessment of the vulnerability of any feature or appurtenance to damage from embankment deformation. This must include an evaluation of the potential of dam failure from a damaged feature or appurtenance. As a check on analysis results, compare estimated performance to observed performanc

Technical Release 210-60 Earth Dams and Reservoirs...TR 210-60 Earth Dams and Reservoirs i...

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